FIG. 15 and FIG. 16 illustrate an example of conventional construction of a rolling bearing unit for supporting a wheel of an automobile such that the wheel rotates freely with respect to the suspension. In the rolling bearing unit 1 that is illustrated in FIG. 15 and FIG. 16, an outer ring 2, which is a stationary-side ring that does not rotate during operation and that is supported by and fastened to the suspension, and a hub 3, which is a rotational-side ring that rotates during operation and is fastened to and supports the wheel, are arranged so as to be concentric with each other. A plurality of balls 6, which are rolling elements are arranged between each of two rows of outer raceways 4, which are each stationary-side raceways, that are formed around the inner circumferential surface of the outer ring 2, and two rows of inner raceways 5, which are each rotational-side raceways, that are formed around the outer circumferential surface of the hub 3. The balls 6 are held by a retainer 7 so as to be able to roll freely. The hub 3 comprises a hub body 18 and an inner ring 19 that is fastened around the inside end in the axial direction of the hub body 18. In this specification, the “outside” in the axial direction means the outside in the width direction of the vehicle when the bearing is assembled in the vehicle, or in other words, indicates the left side in each of the drawings. On the other hand, the “inside” in the axial direction means the center side in the width direction of the vehicle, or in other words, indicates the right side in each of the drawings. Of the inner raceways 5, the inner raceway 5 that is on the outside in the axial direction is formed around the outer circumferential surface in the middle section in the axial direction of the hub body 18, and the inner raceway 5 that is on the inside in the axial direction is formed around the outer circumferential surface of the inner ring 19. In the case of a bearing that is used in a heavy vehicle, rollers may be used instead of balls as the rolling elements.
The openings on both end sections of the annular space 8 between the inner circumferential surface of the outer ring 2 and the outer circumferential surface of the hub 3, where the balls 6 are located, are respectively sealed all the way around by a seal ring 9 and a combined seal ring 10. As a result, it is possible to prevent grease leakage from the annular space 8 to the external space, and it is possible to prevent foreign matter such as moisture, dust and the like that exists in the external space from entering inside the annular space 8. The seal ring 9 comprises a circular ring-shaped seal member 11 made of an elastic material and a circular ring-shaped metal insert 12 that reinforces the seal member 11. With this metal insert 12 fastened around the outside end section in the axial direction of the outer ring 2 by a tight fit, the tip end edges of a plurality of seal lips of the seal member 11 are brought in sliding contact all the way around the outer circumferential surface of the middle section in the axial direction of the hub body 18.
Moreover, the combined seal ring 10 comprises a slinger 13 and a seal ring 14. The slinger 13 is formed into a circular ring-shape around the entire circumference with an L-shaped cross section by punching or bending metal plate using a stamping process such as a burring process, and comprises: a cylindrical section 15 that extends in the axial direction; a circular ring section 16 that is bent outward in the radial direction from the edge on the inside end in the axial direction of the cylindrical section 15, and a curved section 17 having an arc shaped cross section that exists in the connecting section between the cylindrical section 15 and the circular ring section 16. This kind of slinger 13 is fastened to the inner ring 19 by fitting the cylindrical section 15 onto the cylindrical shaped slinger fitting surface 20 that is formed around the outer circumferential surface of the inside end in the axial direction of the inner ring 19 by a tight fit (press fit). Moreover, the seal ring 14 comprises a circular ring-shaped seal member 21 that is made of an elastic material, and a circular ring-shaped metal insert 22 that reinforces the seal member 21. This kind of seal ring 14 is fastened to the outer ring 2 by fitting the cylindrical section 23 that is formed on the outer peripheral edge section of the metal insert 22 into the cylindrical shaped seal ring fitting surface 24 that is formed around the inner circumferential surface on the inside end section in the axial direction of the outer ring 2 by a tight fit (press+fit). In this state, the tip end edges of a plurality of seal lips of the seal member 21 are brought into sliding contact all the way around the circumference of the surface of the slinger 13. Chamfered sections 25, 26 having an arc shaped cross section are formed on the edge section on the inside end in the axial direction of the outer circumferential surface of the inner ring 19 and the edge section on the inside end in the axial direction of the inner circumferential surface of the outer ring 2. These chamfered sections 25, 26 function as guides when the press fitting of the cylindrical sections 15, 23 onto the slinger fitting surface 20 and into the seal ring fitting surface 24 from the inside in the axial direction is carried out.
When manufacturing this kind of rolling bearing unit 1, the outer ring 2 and hub 3 (hub body 18, inner ring 19) are made by going through a plurality of processes such as a turning process for obtaining a specified shape and dimensions, heat treatment for increasing hardness and toughness, and grinding for preparing the surface precision and surface roughness of the outer raceways 4, inner raceways 5, seal ring fitting surface 24 and slinger fitting surface 20. Of these, as a method for efficiently performing grinding, JP2006-329322(A) discloses a method as illustrated in FIG. 17 of grinding both the inner raceway 5 on the inside in the axial direction and the slinger fitting surface 20 at the same time by a formed grindstone 31, and a method as illustrated in FIG. 18 of grinding both the outer raceway 4 on the inside in the axial direction and the seal ring fitting surface 24 at the same time using a formed grindstone 35.
Incidentally, when performing grinding by the methods illustrated in FIG. 17 and FIG. 18 and described above, there is a possibility that the following problems may occur. That is, when manufacturing the inner ring 19, the slinger fitting surface 20 and the chamfered section 25 that are adjacent to each other are formed by turning before heat treatment such that in the state before heat treatment the edges of the respective ends are smoothly continuous with each other. The same is also true for the seal ring fitting surface 24 and the chamfered section 26 when manufacturing the outer ring 2.
However, after undergoing heat treatment, in the state after grinding has been performed, the continuous section between the edges of the end sections is no longer smooth. In other words, in the inner ring 19, of the slinger fitting surface 20 and the chamfered section 25, only the slinger fitting surface 20 is ground, and in the outer ring 2, of the seal ring fitting surface 24 and the chamfered section 26, only the seal ring fitting surface 24 is ground, so an edge section comprising a sharp corner section (a non-differential corner section in the cross-sectional shape) is formed in the joining section 27 between the ground slinger fitting surface 20 and the turned chamfered section 25, and in the joining section 32 between the ground seal ring fitting surface 24 and the turned chamfered section 26.
Particularly, because the difference in the thickness at each site of the inner ring 19 and the outer ring 2 is large, it becomes easy for the difference in the amount of thermal expansion during heat treatment at each site to become large, and a loss of roundness after heat treatment also becomes large, so it becomes necessary to take away large cutting stock during the grinding processes above. As a result, the tip ends of the edge sections in the joining section 27 and joining section 32 become sharp, and are formed unevenly around the circumference. Therefore, when press fitting the cylindrical section 15 of the slinger 13 over the slinger fitting surface 20, and the cylindrical section 23 of the seal ring 14 over the seal ring fitting surface 24 from the inside in the axial direction, the joining section 27 and joining section 32 do not function properly as guides for putting the cylindrical section 15 and cylindrical section 23 onto the slinger fitting surface 20 and the seal ring fitting surface 24, so there is a possibility that the slinger 13 and seal ring 14 will be tilted, or that the inner circumferential surface of the cylindrical section 15 and the outer circumferential surface of the cylindrical section 23 will be damaged in the axial direction by the edge sections.
Particularly, in the case of the slinger 13, the rigidity of the cylindrical section 15 becomes higher closer to the circular ring section 16. Therefore, when the tight fit of the cylindrical section 15 on the slinger fitting surface 20 is uniformly set all around, the surface pressure between slinger fitting surface 20 and the cylindrical section 15 becomes higher closer to the circular ring section 16. This means that the width and depth of damage in the axial direction to the inner circumferential surface of the cylindrical section 15 during press fitting becomes greater in the portion nearer to the circular ring section 16, and is the greatest in the portion the closest to the circular ring section 16, or in other words, at the inside end section in the axial direction of the inner circumferential surface of the cylindrical section 15. Moreover, as damage is formed, chips that are removed from the inner circumferential surface of the cylindrical section 15 act as built-up edges that become larger as press fitting proceeds, and the cross-sectional area of the damage also becomes large. As a result, the damage penetrates through the inside end section in the axial direction of the inner circumferential surface of this cylindrical section 15, and it becomes easy for damage, whose cross-sectional area becomes larger going toward the inside in the axial direction, to be formed along the entire length in the axial direction.
In the case of the seal ring 14 as well, as the press fitting described above proceeds, there is a possibility that damage will occur along the entire length in the axial direction of the outer circumferential surface of the cylindrical section 23 of the metal insert 22, or in other words, in the portion that fits with and comes in contact with the seal ring fitting surface 24. However, in the case of the conventional construction described above, the outer edge section of the seal member 21 that is fastened to the metal insert 22 comes in elastic contact all around the inner circumferential surface on the inside end section in the axial direction of the outer ring 2. Therefore, even though damage occurs along the entire length in the axial direction of the outer circumferential surface of the cylindrical section 23, the outer edge section around the seal member 21 is able to maintain the seal between the cylindrical section 23 and the seal ring fitting surface 24 to a certain extent.
On the other hand, in the case of the slinger 13, unlike the case of the seal ring 14, there is no seal member. Therefore, as the press fitting described above proceeds and damage is formed along the entire length in the axial direction on the inner circumferential surface of the cylindrical section 15 of the slinger 13, this damage causes the seal at the area of fit between the cylindrical section 15 and the slinger fitting surface 20 to be lost. In other words, due to this damage, it becomes easier for grease inside the annular space 8 to leak out into the external space, and for foreign matter such as moisture, dust and the like that exists in the external space to enter into the annular space 8.